Fuel trim units are commonly used to control the fuel entering a combustion chamber in a multi-chamber combustor of an industrial turbine, for example a gas turbine. Generally, these units match the combustion airflow entering each combustion chamber such that the fuel-air mixture minimally produces, upon burning, nitrous oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHC). In order to minimize CO and UHC, and achieve overall greater efficiency, it is desirable to increase the combustion temperature within the turbine. However, the oxidation of NOx in turbines increases dramatically with the increase in combustion temperatures.
One common method for reducing NOx is to lower the combustion temperature in a turbine system, or make the fuel-air ratio lean. However, if the fuel-air mixture is too lean, then ‘lean-burn-out’ occurs, and undesirable emissions increase dramatically. Therefore, a careful balance must be struck between (1) increasing the efficiency (minimizing UHC and CO) by increasing combustion temperature, and (2) decreasing the combustion temperature to minimize NOx, or thinning the fuel-air ratio such that lean-burn-out occurs versus maximizing power output by increasing the fuel-air ratio.
It is extraordinarily difficult to achieve uniform temperature and pressure distribution in multiple combustion chambers of an industrial gas turbine. Variations in the airflow in each combustion chamber make it difficult to maintain constant fuel-air ratios in all combustion chambers.
These various teachings known to those skilled in the art are described in the following patents. U.S. Pat. No. 4,292,801 to Wilkes et al. discloses a 2-stage gas turbine capable of reduced emissions of nitrous oxides. U.S. Pat. No. 5,319,931 to Beebe et al. discloses a fuel trim system for a multi-chamber gas turbine engine. U.S. Pat. No. 5,423,175 to Beebe et al. discloses a fuel trim system for a multi-chamber gas turbine system in which sensor inputs with a fuel flow rate as well as the dynamic pressure in each combustion chamber and the turbine exhaust temperature are measured and accounted for in varying the fuel-air mixture.
Although these patents disclose fuel trim systems including multiple manifolds for supplying fuel nozzles with fuel in each combustion chamber of a multi-chamber gas turbine, none of these references teaches, suggests, or discloses to one skilled in the art a fuel trim valve for controlling each fuel nozzle in each combustion chamber. Furthermore, it is not obvious to provide each fuel nozzle in each combustion chamber with a fuel trim valve as this is extraordinarily difficult because: 1) there is limited piping room in a gas turbine engine to incorporate a fuel trim valve for each fuel nozzle; 2) in order to increase efficiency it is necessary to incorporate multiple fuel manifolds so that the pressure drop across each fuel trim valve is within a small uniform range; and 3) adjusting each of the fuel trim valves in each combustion chamber is a Herculean task.
Therefore, what are needed are systems and methods to control the fuel-air ratio of a multi-chamber gas turbine by employing a fuel trim valve with each fuel nozzle.
What are also needed are systems and methods to control the fuel-air mixture in each combustion chamber of a multi-chamber gas turbine such that the combustion chamber pressure oscillations, NOx, UHC, and CO are minimized for a given energy output for the gas turbine.
What are further needed are simple systems and methods for adjusting each fuel valve in each combustion chamber, such that the fuel-air ratio in each combustion chamber can be optimized to minimize combustion chamber pressure oscillations, NOx, UHC, and CO for the gas turbine.